III-V semiconductor compounds and other mixed semiconductors, as opposed to type IV semiconductors such as silicon, often lack a stable passivating oxide. As a consequence, it is often difficult to build reliable and stable insulated gate field effect transistors (IGFETs) and other devices employing insulated gate electrodes because of instabilities associated with the semiconductor surface and/or the dielectrics used for the insulated gates. This problem is observed for GaAs devices employing gallates such as gadolinium oxides, gallium oxides, and combinations thereof, for the gate dielectric and surface passivation. For example, gallium oxide has been shown to exhibit a low interfacial density and to unpin the Fermi level on GaAs, and high transconductance transistors have been demonstrated with gadolinium gallium oxide/gallium oxide insulated gate dielectric. However, such devices can exhibit undesirably high series ON-resistance, partly due to high sheet resistance e.g., equal or greater than ˜500 Ohms per square, for example, resulting from bulk dielectric charge trapping that can effectively immobilize or reduce the number mobile carriers available in the channel of the device for forward conduction. Charge trapping in the insulating dielectric can be time dependent so that initial high device performance can degrade over time. Even though various surface treatments have been developed to ameliorate such problems, the improvements are usually not as large as desired and/or the improvements are only temporary and the devices often revert, for example, to their pre-treatment conditions of undesirably high sheet resistance. Thus, a need continues to exist for improved structures and methods for non-type IV insulated gate field effect devices, especially those employing GaAs and other III-V binary or ternary semiconductor compounds and gallate insulating gate dielectric layers, that provide stable and long-lasting reduction in the source-drain and/or substrate leakage and/or that lower the device sheet resistance.